stream: convert string to Buffer when calling `unshift(<string>)`

`readable.unshift` can take a string as an argument, but that
string wasn't being converted to a Buffer, which caused a
<TypeError: Argument must be a buffer> in some cases. Also if a
string was passed, that string was coerced to utf8 encoding.

A second optional argument `encoding` was added to `unshift` to
fix the encoding issue.

Fixes: https://github.com/nodejs/node/issues/27192
PR-URL: https://github.com/nodejs/node/pull/27194
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

1 files changed, 18 insertions, 14 deletions

diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
index 163a042f08..d6db718875 100644
--- a/lib/_stream_readable.js
+++ b/lib/_stream_readable.js
@@ -207,13 +207,28 @@ Readable.prototype._destroy = function(err, cb) {
 // similar to how Writable.write() returns true if you should
 // write() some more.
 Readable.prototype.push = function(chunk, encoding) {
-  const state = this._readableState;
-  var skipChunkCheck;
+  return readableAddChunk(this, chunk, encoding, false);
+};
+
+// Unshift should *always* be something directly out of read()
+Readable.prototype.unshift = function(chunk, encoding) {
+  return readableAddChunk(this, chunk, encoding, true);
+};
+
+function readableAddChunk(stream, chunk, encoding, addToFront) {
+  debug('readableAddChunk', chunk);
+  const state = stream._readableState;
+
+  let skipChunkCheck;
 
   if (!state.objectMode) {
     if (typeof chunk === 'string') {
       encoding = encoding || state.defaultEncoding;
-      if (encoding !== state.encoding) {
+      if (addToFront && state.encoding && state.encoding !== encoding) {
+        // When unshifting, if state.encoding is set, we have to save
+        // the string in the BufferList with the state encoding
+        chunk = Buffer.from(chunk, encoding).toString(state.encoding);
+      } else if (encoding !== state.encoding) {
         chunk = Buffer.from(chunk, encoding);
         encoding = '';
       }
@@ -223,17 +238,6 @@ Readable.prototype.push = function(chunk, encoding) {
     skipChunkCheck = true;
   }
 
-  return readableAddChunk(this, chunk, encoding, false, skipChunkCheck);
-};
-
-// Unshift should *always* be something directly out of read()
-Readable.prototype.unshift = function(chunk) {
-  return readableAddChunk(this, chunk, null, true, false);
-};
-
-function readableAddChunk(stream, chunk, encoding, addToFront, skipChunkCheck) {
-  debug('readableAddChunk', chunk);
-  const state = stream._readableState;
   if (chunk === null) {
     state.reading = false;
     onEofChunk(stream, state);